1. Field
This invention is concerned generally with thixotropic gel-like compositions which are used to assist in the separation and partitioning of whole blood into serum and clot portions. Specifically, the invention is concerned with methods of preparing such compositions in a manner which permits an effective and convenient control of viscosity.
2. Prior Art
The specific gravity of whole human blood is generally within the range of about 1.048 to 1.066. It has long been known that such blood can be readily centrifuged to effect a separation of the blood into two major components--a lighter serum portion having a specific gravity within the range of about 1.026 to 1.031 and a heavier clot portion, consisting mainly of red blood cells, having a specific gravity within the range of about 1.092 to 1.095. Such separations of blood into its two major portions are commonly used to facilitate physical and chemical analyses of blood. Such analyses are helpful in the diagnosis and prognosis of many human ailments.
With the advent of modern techniques for the analyses of various physical and chemical sub-components of blood, there has been a general recognition that simple centrifugation of blood into its two major components does not necessarily effect an ideal separation for analytical purposes. For example, even though simple separation yields a gross separation of whole blood into serum and clot portions, there still exists an interface between the separated portions which, especially with time, results in diffusion of various sub-components of one separated portion into the other. Such diffusion can affect the accuracy of various analyses.
In recent years, considerable efforts have been made to modify simple centrifugation techniques so that the formation of a serum-clot interface is avoided. For example, it is now well known that certain gel-like, essentially inert, thixotropic substances, having a specific gravity intermediate those of the serum and clot portions, can be used to assist in the separation and partitioning of serum and clot portions. One such substance comprises a viscous gel-like material consisting of a silicone fluid having inert siliceous particles dispersed therein in an amount sufficient to yield a specific gravity in the range of about 1.030 to about 1.050. When whole blood, contained in a test tube, is centrifuged in the presence of such a substance, the gel-like material, because of its thixotropic nature and specific gravity, tends to migrate to a position intermediate those of the serum and clot portions and ultimately assume a configuration and position which discourages and prevents the formation of a serum-clot interface by acting as a physical and chemical barrier.
Various examples of such silica-silicone fluid composition are well known in the art and described more fully, for example, in U.S. Pat. No. 3,780,935 to Lukacs and Jacoby; U.S. Pat. No. 3,852,194 to Zine; and patent application Ser. No. 532,946 cited above as a related application. In the two cited patents, preferred compositions consist essentially of a silicone fluid such as a dimethylpolysiloxane having very fine silica particles dispersed therein to achieve a desired specific gravity. More recently, in patent application Ser. No. 532,946, it was disclosed that such compositions can be stabilized to avoid "wet out" through the addition of relatively minute amounts of certain polysiloxane-polyoxyalkyl copolymers which act as network formers between the dispersed silica particles.
Very generally, the expression "wet out" refers to the tendency of such compositions to lose viscosity with time. Such loss in viscosity results in a loss in the gel-like nature of the composition and the relative firmness of the composition is lessened. A very undesirable effect of wet out occurs when an attempt is made to pour off the upper serum portion of separated blood contained in a test tube. For example, as long as the test tube containing separated blood is maintained in an essentially vertical position a wetted out barrier generally poses no particular problem. However, as the tube is tilted to pour off the upper serum portion, a wetted out barrier has a tendency to slump, thereby disturbing the previously maintained seal between the serum and clot portions. As the seal is disturbed, there forms an interface between the serum and clot portions, thus permitting diffusion of various sub-components of blood and adversely affecting certain analyses. It is also recognized that the undesired slumping due to wet out can occur even if a test tube containing the separated blood is jarred. Hence, as described in greater detail in patent application Ser. No. 532,946, the use of certain network formers in silica-silicone fluid compositions provides a practical method for stabilizing such compositions against wet out (or viscosity loss) for periods of up to and greater than one year.
The occurrence of wet out is thought to be due to a complete interaction of the silica surfaces with the silicone fluid over a period of time. Although the silica particle fillers are often described as being essentially inert, it is known that the surfaces of such particles have available hydroxyl groups capable of interaction with the silicone fluid. The use of certain network formers tends to minimize such surface interaction and, thus, preclude wet out of the particles by assuring that the surfaces of the particles are not completely enveloped by the silicone fluid. It is thought that the network formers tend to compete with the silicone fluid for surface sites on the particles.
In using network formers to stabilize silica-silicone fluid compositions, however, it is recognized that the addition of the network formers has a dramatic effect in increasing the viscosity of the composition. Hence, very small amounts of network former must be used to achieve a stable and controlled viscosity in a range useful for blood separating applications; e.g., about 200,000 to 600,000 centistokes, preferably 350,000 to 450,000 centistokes. As can be seen from Ser. No. 532,946, the amounts of certain polysiloxane-polyoxyalkyl copolymer network formers used to stabilize such silica-silicone fluid compositions were in the range of about 0.0173 to 0.0117 parts by weight. Because such small amounts are used, it is a common practice to carefully titrate the network former into the silica-silicone fluid mixture to achieve a viscosity within a given range.
Although one way of avoiding the tight control of such small amounts of network former involves the use of "methylated" silica particles (having less surface sites available for interaction with the network former and thereby permitting use of larger amounts), it can be appreciated that commercially available "methylated" silica is, in fact, only partially methylated, and it would be fortuitous to obtain a commercially available silica with exactly the desired degree of methylation for this application. It can be appreciated that the preparation of the overall composition involves at least three or more processing steps--e.g., preparing or acquiring a partially methylated silica, mixing it with a silicone fluid, a possible degassing step, a possible dehydration step, and also the addition of an appropriate amount of network former if wet out is to be avoided.
I have found that the processing steps and costs associated with the preparation of such silicone fluid-silica compositions can be effectively reduced and that it is possible to add conveniently varying amounts of network formers to stabilize such compositions. My method of preparing such compositions are described in detail below.